The Coking Process
Coking has been practiced for many years. The process involves the exposure of a feed stream to heat, resulting in thermal cracking of heavy liquid hydrocarbons in the stream to produce gas, liquid streams of various boiling ranges, and coke.
Various processes for the production of coke are known in the art. In a delayed coking process, a petroleum fraction is heated to coking temperatures and then fed into a coke drum under conditions that initiate thermal cracking. Following the cracking off of lighter constituents, polymerization of the aromatic structures occurs, depositing a porous coke mass in the drum.
In a typical delayed coking process, residual oil is heated by exchanging heat with the liquid products from the process and then fed into a fractionating tower where any light products that might remain in the residual oil are distilled out. The oil is then pumped through a furnace, where it is heated to the required coking temperature. From the furnace, the hot oil is discharged into the bottom of the coke drum. The oil undergoes thermal cracking and polymerization for an extended period, resulting in the production of hydrocarbon vapors and porous carbonaceous coke that remains in the drum. The vapors leave the top of the drum and are returned to the fractionation tower, where they are fractionated into the desired cuts. This process is continued until the drum is substantially full of porous coke. Residual oil feed is then typically switched to a second parallel drum, while steam is introduced through the bottom inlet of the first drum to quench the coke.
The steam strips out the remaining uncracked oil in the drum. During the early stage of steaming, the mixture of water and oil vapors continues to pass to product recovery, as during the coking stage. Thereafter, the effluent from steaming is diverted to blow-down facilities, where it is condensed and transferred to settling basins. In the settling basins, oil is skimmed from the surface of the water.
After steam cooling to about 700.degree.-750.degree. F., water is introduced to the bottom of the coke drum to complete the quench. The first portions of water are, of course, vaporized by the hot coke. The resultant steam plus oil vapor is passed to blow-down for condensation and skimming to separate oil. Water addition is continued until the drum is completely filled with water. For a period thereafter, water is introduced to overflow the drum with effluent sent to settling equipment for removal of entrained oil, etc.
The water settling system also receives water from other operations in the coker facility as later described. The clarified water produced by the settling system provides the water for quench and for recovery of coke from the drum. Coke recovery proceeds by removal of top and bottom heads from the drum and cutting of the coke by hydraulic jets. First, a vertical pilot hole is drilled through the mass of coke to provide a channel for coke discharge through the bottom opening. Then a hydraulic jet is directed against the upper surface of the coke at a distance from the central discharge bore, thereby cutting the coke into pieces. The pieces drop out of the coke drum through the pilot hole. The cutting jet traverses the drum until the coke bed is completely removed.
The coke leaving ranges in size from large lumps to fine particles. To a considerable extent, the fines are separated from the larger pieces as the coke discharges into slotted bins or hopper cars, with the water draining off through the slots. This dispersion of fines in water is processed to recover the fines as solid fuel, and the water returns to the system for use in quenching and cutting.
In a flexicoking process, a material stream circulates continuously between a reactor and a heater. More specifically, a feed stream is fed into a fluidized bed, along with a stream of hot recirculating material. From the reactor, a stream containing coke is circulated to a heater vessel, where it is heated. The hot coke stream is sent from the heater to a gasifier, where it reacts with air and steam. The gasifier product gas, referred to as coke gas, containing entrained coke particles, is returned to the heater and cooled by cold coke from the reactor to provide a portion of the reactor heat requirement. A return stream of coke sent from the gasifier to the heater provides the remainder of the heat requirement. Hot coke gas leaving the heater is used to generate high-pressure steam before being processed for cleanup. Coke is continuously removed from the reactor.
In a fluid coking process, a fluidized bed reactor is used in conjunction with a burner to provide continuous coke production. The feed stream is introduced into a scrubber, where it exchanges heat with the reactor overhead effluent and condenses the heaviest fraction of the hydrocarbons leaving the top of the reactor. The total reactor feed, including both the fresh feed and the recycle condensed in the scrubber, is injected into a bed of fluidized coke in the reactor. The coke is laid down on the fluidized coke particles, while the hydrocarbon vapors pass overhead into the scrubber. The reactor overhead is scrubbed for solids removal and the high boiling material is condensed and recycled to the reactor. The lighter hydrocarbons are sent from the scrubber to conventional fractionation, gas compression, and light ends recovery units.
Heat required to maintain the reactor at coking temperature is supplied by circulating coke between the reactor and the burner. A portion of the coke produced in the reactor is burned with air to satisfy the process heat requirements. The excess coke is withdrawn from the burner and sent to storage.
Sludge Disposal
Many refineries, chemical plants, waste water treatment plants and other such industrial and municipal facilities generate waste products in the course of their operation. For example, in the refining of petroleum there are produced waste products or streams such as heavy oil sludges, biological sludges from waste water treatment plants, activated sludges, gravity separator bottoms, storage tank bottoms, oil emulsion solids including slop oil emulsion solids and dissolved air flotation (DAF) float from flocculation separation processes, etc. The disposal of these waste products can create difficult and expensive environmental problems primarily because the waste streams are not readily amenable to conversion to more valuable, useful or ecologically innocuous products.
Several methods have been proposed for dealing with the disposal, in an economical and environmentally acceptable fashion, of waste products such as petroleum refinery sludges and other such waste products. One proposal for dealing with petroleum sludges is disclosed in U.S. Pat. No. 3,917,564, which discloses a process in which sludges and other wet by-products of industrial and municipal activities are added to a delayed coker as an aqueous quench medium during the quench portion of the delayed coking cycle. The combustible solid portions of the by-product become a part of the coke, and the non-combustible solids are distributed throughout the mass of the coke so that the increase in the ash content of the coke is within commercial specifications, especially for fuel grade coke products.
Another patent relating to disposal of refinery waste solids in a coker quench stream is U.S. Pat. No. 5,443,717, which discloses pretreating the sludge before injecting it into the main quench stream. More particularly, '717 patent discloses passing the waste stream (sludge) through a centrifuge, where it is separated into an oil stream, a water stream and a wet sediment stream. The wet sediment stream is in turn passed through a dewatering apparatus and the dewatered solids are then fed into the main quench stream of the coker.
Still another process is disclosed in U.S. Pat. No. 4,666,585, which discloses a process in which petroleum sludges are recycled by adding them to the feedstock of a delayed coker before the quenching cycle so that the sludge, together with the feed, is subjected to delayed coking. This process has the desirable aspect of subjecting the combustible portion of the sludge to the high coking temperatures so that either the conversion to coke or the distillation of residual hydrocarbon products takes place. The presence of water in the sludge tends to lower the temperature in the coker unless compensation is made for this factor, for example, by increasing the operating temperature of the coking furnace. This in turn may decrease the yield of the more desirable liquid product from the delayed coking process. In addition, because the sludge contains large amounts of water and oil, the amount of sludge that can be added to the coker feed is limited by the presence of the relatively large amount of water in the sludge. It has been calculated that for every ton of water that passes through the coker unit, coker production is reduced by approximately 4-1/2 tons of coker feed. Likewise, oil in the waste is unnecessary for a coker unit. It has been calculated that each ton of oil passing through the coker unit reduces the coker feed by approximately 1-1/2 tons. As described in the '585 patent, the amount of sludge in the stream is limited to a maximum of 2 weight percent.
Another proposal for dealing with petroleum sludges is disclosed in U.S. Pat. No. 4,874,505, in which oily sludges and other refinery waste streams are segregated into a high oil content waste that is injected into a delayed coking unit during the coking phase of the cycle and a high water content waste that is injected during the quenching phase of the delayed coking cycle. This process purportedly increases the capacity of the delayed coker to process refinery wastes and sludges and has the potential for improving the quality of the resulting coke obtained from the process. Using this process, refinery sludges can be added at a rate of up to about 2 bbl/ton of coke produced. The separation process adds an additional process step and neither stream is sufficiently tailored to avoid undesirably affecting the coker operation. For example, the water content of the stream entering the coker is disclosed to be 25%, again resulting in a severe reduction of coker efficiency. U.S. Pat. No. 5,009,767, discloses a process similar to the '505 patent, with the modification that the high oil content sludge is filtered to remove water prior to being introduced into the delayed coking unit during the coking phase of the cycle.
While the above processes are somewhat effective for disposing of waste products such as refinery sludges, in general they are not wholly satisfactory. For example, there is often a significant loss of valuable oil (organics), which is absorbed in the coke or collected in the blow-down system. With quench cycle injection of raw oil sludges, there is a tendency for oily build-up to occur in the coke drum, causing the volatile combustible matter (VCM) levels in the coke to be objectionably high. Likewise, when sludge is incorporated in the coker feedstock, both oil and water in the sludge adversely affect the efficiency of the system by reducing the production of coke.
Hence it is desirable to provide a method that allows addition of a refinery waste stream or sludge to the coking process without encountering the disadvantages heretofore associated with such additions. The present invention significantly minimizes the disadvantages of the prior art.